If you can, will it keep the special moves and costume? Thanks in advance! Also I would like a source.
No, they cannot be transferred past Omega Ruby/Alpha Sapphire. They were not programed for an unknown reason in the other games. They are exclusive for Omega Ruby/Alpha Sapphire.
Pokemon Bank doesn't let you transfer special Pokemon, such as Cosplay Pikachu, to future games or games that it doesn't exist in.
Hope this helped!